Method for stripping a component

ABSTRACT

A method for stripping a component, in particular a gas turbine component, to completely or partially remove a multilayer or sandwich antiwear coating from the surface of the component, the antiwear coating including at least one relatively hard ceramic layer and at least one relatively soft metallic layer is disclosed. According to the present invention, in order to remove the multilayer or sandwich antiwear coating, the component is placed in a bath of an alkaline electrolyte, the component placed in the electrolyte being stripped at a current density of between 1 A/dm 2  and 20 A/dm 2 .

This claims priority to German Patent Application DE 10 2007 022 832.7,filed May 15, 2007 through international application PCT/DE2008/000756,filed May 2, 2008, the entire disclosures of which are herebyincorporated by reference herein.

The present invention relates to a method for stripping a component, inparticular a gas turbine component, to completely or partially remove amultilayer or sandwich antiwear coating from the surface of thecomponent, the antiwear coating including at least one relatively hardceramic layer and at least one relatively soft metallic layer.

BACKGROUND OF THE INVENTION

To provide gas turbine components, such as rotor blades, withoxidation-resistance, corrosion-resistance or also erosion-resistance,special antiwear coatings are applied to the surfaces thereof. Duringoperation, gas turbine components are subject to wear or can becomedamaged in some other way. Repairing the damage typically requireslocalized, partial, or also complete removal or ablation of the antiwearcoating from the component to be repaired. The process of removing orablating coatings is also described as stripping. One distinguishesamong the different stripping methods according to whether the coatingsare removed mechanically, chemically or electrochemically.

Antiwear coatings typically take the form of what is known as multilayercoatings, which are composed of a plurality of layers alternatelydeposited on the gas turbine component. Thus, an antiwear coating in theform of a multilayer coating may include, for example, a relatively softmetallic layer and a relatively hard ceramic layer, a multiplicity ofsaid layers being alternately deposited one above another on the gasturbine component. In practice, there are known antiwear coatings inwhich more than two different layers are alternately deposited one aboveanother on the gas turbine component, such as, for example, multilayercoatings composed of four layers alternately deposited one above anotheron the gas turbine component, namely a first metallic layer which isadapted to the material composition of the gas turbine component andwhich is therefore relatively soft, a metallic layer which is composedof a metal alloy material and is also relatively soft, a third,relatively hard-grade metal-ceramic layer, and a fourth relatively hardceramic layer.

The prior art has not yet disclosed a method that would allow multilayerantiwear coatings to be effectively removed without the risk of damageto the base material of the gas turbine component.

SUMMARY OF THE INVENTION

In view of the above, it is an object of the present invention toprovide a novel method for stripping a gas turbine component.

The present invention provides a method for stripping a gas turbinecomponent to completely or partially remove a multilayer or sandwichantiwear coating from the surface of the component, the antiwear coatingincluding at least one relatively hard ceramic layer and at least onerelatively soft metallic layer.

According to the present invention, in order to remove the multilayer orsandwich antiwear coating, the component is placed in a bath of analkaline electrolyte, the component placed in the electrolyte beingstripped at a current density of between 1 A/dm² and 20 A/dm².

The method of the present invention for stripping a component has theadvantage of allowing residue-free removal of multilayer coatings fromthe surface of the component to be stripped without the risk of the basematerial thereof being attacked and thereby damaged. In particular,titanium-based alloys and nickel-based alloys are not damaged during thestripping process according to the present invention. Another advantageof the method of the present invention is that it allows components tobe stripped in a single bath of the alkaline electrolyte. Thus, themethod according to the present invention for stripping a componenteliminates the need to provide several different baths.

Preferably, the component placed in the electrolyte is anodicallyprocessed at room temperature.

In an advantageous embodiment of the present invention, the bathcontains, in addition to the alkaline electrolyte, a surfactant toreduce surface or interfacial tension. In particular, a sodium hydroxidesolution or a potassium hydroxide solution is used as the electrolyte.The surfactant used is especially a fluorinated surfactant, saidsurfactant being used in an amount to reduce the surface or interfacialtension to a value between 20 dynes and 50 dynes.

DETAILED DESCRIPTION

An exemplary embodiment of the present invention is described in greaterdetail below.

The method of the present invention is used for stripping components,such as gas turbine components, which are coated with multilayerantiwear coatings composed of at least two different layers arrangedalternately one above another, namely of relatively hard ceramic layersand relatively soft metallic layers arranged alternately one aboveanother.

In accordance with the method of the present invention, in order toremove a multilayer antiwear coating from a component, and thus to stripthe component, the component to be stripped is placed in a bath of analkaline electrolyte. The component placed in the alkaline electrolyteis then stripped by applying an electric current having a currentdensity of between 1 A/dm² and 20 A/dm².

Accordingly, in accordance with the present invention, stripping isaccomplished electrochemically. In this process, the component to bestripped is mainly processed anodically; i.e., it mainly functions as ananode during stripping. During the stripping process, the component tobe stripped can also be used as a cathode at defined intervals for shortperiods of time in order to increase the stripping efficiency.

Preferably, the stripping of the component placed in the alkalineelectrolyte is performed at a current density of between 1 A/dm² and 5A/dm², and preferably at room temperature.

Preferably, sodium hydroxide solution or potassium hydroxide solution isused as the alkaline electrolyte.

In order to reduce the surface or interfacial tension during stripping,it is preferred to use a bath which contains a surfactant in addition tothe alkaline electrolyte. The surfactant is preferably a fluorinatedsurfactant.

The amount of the surfactant is selected to reduce the surface orinterfacial tension to a value between 20 dynes und 50 dynes, inparticular to a value between 25 dynes und 35 dynes.

If the component is to be stripped only in specific surface regions, theportions or regions of the component that are intended not to bestripped are covered with a wax-containing or wax-like material prior tostripping. This is useful, for example, when a gas turbine blade is tobe stripped only in the region of its blade, but not in the region ofits root. In such a case, the blade root is covered with awax-containing or wax-like material prior to placing the gas turbineblade in the bath.

The method of the present invention enables effective, low-stressstripping of multilayer coatings from components. Stripping isaccomplished electrochemically, it being sufficient to use a single bathof an alkaline electrolyte. There is no risk of the base material of thecomponent to be stripped being eroded and thus damaged.

1-11. (canceled)
 12. A method for stripping a component, in particular a gas turbine component, to completely or partially remove a multilayer or sandwich antiwear coating from the surface of the component, the antiwear coating including at least one relatively hard ceramic layer and at least one relatively soft metallic layer, the method comprising: in order to remove the multilayer or sandwich antiwear coating, placing the component in a bath of an alkaline electrolyte; and stripping the component placed in the electrolyte at a current density of between 1 A/dm2 and 20 A/dm2.
 13. The method as recited in claim 12, wherein the component placed in the electrolyte is stripped at a current density of between 1 A/dm2 and 5 A/dm2.
 14. The method as recited in claim 12, wherein the component placed in the electrolyte is stripped anodically.
 15. The method as recited in claim 12, wherein the component placed in the electrolyte is stripped at room temperature.
 16. The method as recited in claim 12, wherein the bath contains, in addition to the alkaline electrolyte, a surfactant to reduce surface or interfacial tension.
 17. The method as recited in claim 16, wherein the surfactant is used in an amount to reduce a surface or interfacial tension to a value between 20 dynes und 50 dynes.
 18. The method as recited in claim 17, wherein the surfactant is used in an amount to reduce the surface or interfacial tension to a value between 25 dynes und 35 dynes.
 19. The method as recited in claim 16, wherein the surfactant is a fluorinated surfactant.
 20. The method as recited in claim 12, wherein the alkaline electrolyte is a sodium hydroxide solution.
 21. The method as recited in claim 12, wherein the alkaline electrolyte is a potassium hydroxide solution.
 22. The method as recited in claim 12, further comprising covering portions of the component that are intended not to be stripped with a wax-containing or wax-like material prior to placing the component in the bath. 